Revenues - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended			6 Months Ended
	Jan. 29, 2024	Jan. 31, 2024	Mar. 31, 2025	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Contract with customer, liability				$ 327		$ 327		$ 398
Option continuation payment due in the third quarter 2024	$ 100
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress and change in estimate of transaction price	$ 107
Contract with customer, liability, cumulative catch-up adjustment to revenue, earnings Per share, basic (in dollars per share)	$ 1.21
Contract with customer, liability, cumulative catch-up adjustment to revenue, earnings Per share, diluted (in dollars per share)	$ 1.21
Premium from Third Stock Purchase Agreement Amendment	$ 87
Deferred revenues as of January 29, 2024	335
Allocation of transaction price	522
Revenue recognized from amounts in deferred revenue at the beginning of the period				39	$ 28	180	$ 53
Total revenues				39	29	184	54
Capitalized contract cost, amortization						2
Cost sharing receivable				3		3		3
R&D Activities for Domvanalimab
Disaggregation of Revenue [Line Items]
Revenue recognized from amounts in deferred revenue at the beginning of the period						25
Access Rights and Option Continuation Periods
Disaggregation of Revenue [Line Items]
Total revenues				9	9	17	17
Taiho Collaboration Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability				42		42		23
Total revenues				3		7
Third Gilead Collaboration Agreement Amendment, Third Stock Purchase Agreement Amendment And The Second Investor Rights Agreement Amendment
Disaggregation of Revenue [Line Items]
Capitalized contract cost, gross	8
Third Stock Purchase Agreement Amendment
Disaggregation of Revenue [Line Items]
Capitalized contract cost, gross				5		5
Initial Gilead Collaboration Agreement and Subsequent Amendments
Disaggregation of Revenue [Line Items]
Capitalized contract cost, gross				3		3
Gilead | Etrumadenant License and Research and Development Services Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability	129			185		185		133
Contract with customer liability revenue recognized excluding opening balance						250
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress	14
Total revenues				3	10	27	18
Gilead | Quemliclustat, License and R&D services
Disaggregation of Revenue [Line Items]
Contract with customer, liability	130			52		52		132
Contract with customer liability revenue recognized excluding opening balance						200
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress	88
Total revenues				19	7	119	15
Gilead | R&D Activities for Domvanalimab
Disaggregation of Revenue [Line Items]
Contract with customer, liability				24		24		25
Total revenues				3	1	10	2
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in estimate of transaction price	5
Gilead | Access Rights and Option Continuation Periods
Disaggregation of Revenue [Line Items]
Contract with customer, liability	51			63		$ 63		54
Current and future programs exclusive access period						10 years
Contingent milestone payments receivable	200			300		$ 300
Option continuation payment receivable upon sixth anniversary of agreement	$ 100
Gilead | Rights to Certain Studies
Disaggregation of Revenue [Line Items]
Contract with customer, liability				34		34
Gilead | R&D Activities for Inflammation Programs
Disaggregation of Revenue [Line Items]
Contract with customer, liability				27		27		$ 31
Total revenues				$ 2	$ 1	$ 4	$ 1
Gilead | Revenue Benchmark | Customer Concentration Risk
Disaggregation of Revenue [Line Items]
Percentage of revenues				92.00%	100.00%	96.00%	100.00%
STAR-221 Development Activities | Taiho Collaboration Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability					$ 35		$ 35
Payment for option exercise						$ 28
STAR-121 Development Activities | Taiho Collaboration Agreement
Disaggregation of Revenue [Line Items]
Revenue recognized from amounts in deferred revenue at the beginning of the period						$ 16
Payment for option exercise		$ 26
STAR-121 Development Activities | Taiho Collaboration Agreement | Forecast
Disaggregation of Revenue [Line Items]
Revenue recognized from amounts in deferred revenue at the beginning of the period			$ 10